Hi
It happens that the two existing subset tools, not only work on their own, but also can complement each other. The tool of the Calibre editor uses one list of characters for each font and suppresses unused fonts. The
Sigil plugin uses a single common list of characters but produces more compact subsets.
This is what happens when you combine them to make them work successively on the same ePub, begining with the Calibre editor (v. 3.21) and concluding with the Sigil plugin (v. 0.2.0.). This example comes from a rather big book.
The font used is an OpenType fork of Linux Libertine which has been darkened for ebook display using the very nice Sherman script (see at the beginning of this thread).
Looking at the second screenshot, you can see that significant size savings can still be obtained.